2.4 Inorganic Compounds Essential to Human Functioning

Inorganic Compounds in the Human Body

Inorganic compounds like water, salts, acids, and bases don't contain carbon-hydrogen bonds, but they're absolutely essential to how your body functions. They help maintain temperature, enable chemical reactions, conduct nerve impulses, and keep your internal chemistry balanced. This section covers what makes each of these compounds so important at the chemical level.

Inorganic vs Organic Compounds

The distinction here is straightforward. Inorganic compounds lack carbon-hydrogen (C-H) bonds. The main ones you need to know are water, salts, acids, and bases. Organic compounds contain C-H bonds and form the structural and functional basis of living things: carbohydrates, lipids, proteins, and nucleic acids.

Both categories are essential for life, but they play very different roles. Inorganic compounds tend to serve as the environment and medium in which organic molecules do their work.

Water's Life-Supporting Properties

Water makes up about 60% of your body mass, and its molecular structure explains why it's so critical. Water is a polar molecule, meaning oxygen pulls electrons closer to itself, giving it a slight negative charge while the hydrogens carry a slight positive charge. This polarity is the root of nearly all of water's useful properties.

• Solvent ability: Water's polarity lets it dissolve other polar and ionic substances (like salts and glucose). This is why it's often called the "universal solvent." Substances that dissolve in water are hydrophilic ("water-loving"), while those that don't are hydrophobic ("water-fearing").
• Cohesion: Water molecules are attracted to each other through hydrogen bonds. This creates surface tension and enables capillary action, which helps move water through narrow spaces like blood vessels.
• High specific heat capacity: It takes a lot of energy to change water's temperature. This means your body temperature stays relatively stable even when the environment fluctuates.
• High heat of vaporization: Water requires substantial energy to evaporate. When you sweat, the evaporating water carries that energy away from your skin, cooling you effectively.

All of these properties trace back to hydrogen bonds between water molecules. Each water molecule can form up to four hydrogen bonds with its neighbors, and these bonds constantly break and reform.

Functions of Essential Salts

Salts dissociate into ions when dissolved in water, and those ions (called electrolytes) perform several critical functions:

• Electrical conduction: Ions like sodium ($Na^+$), potassium ($K^+$), and calcium ($Ca^{2+}$) carry electrical charges that are essential for nerve impulse transmission and muscle contraction.
• Osmotic balance: Electrolytes regulate water movement between body compartments (intracellular vs. extracellular fluid). This keeps cells properly hydrated and at the right volume.
• Enzyme cofactors: Certain metal ions are required for enzymes to function. For example, iron ($Fe^{2+}$) is part of hemoglobin and enables oxygen transport, while zinc ($Zn^{2+}$) is needed for DNA synthesis enzymes.
• Structural support: Calcium and phosphate combine to form hydroxyapatite, the mineral compound that gives bones and teeth their hardness and strength.

Acids, Bases, and pH Balance

Acids are substances that donate hydrogen ions ($H^+$) to a solution, lowering its pH. Bases accept hydrogen ions, raising the pH.

The pH scale runs from 0 to 14:

• Below 7 = acidic
• Exactly 7 = neutral
• Above 7 = basic (alkaline)

Some body examples to know:

• Hydrochloric acid (HCl) in your stomach creates a highly acidic environment (pH ~1.5–3.5) to break down food and kill bacteria.
• Lactic acid accumulates in muscles during intense exercise.
• Bicarbonate ($HCO_3^-$) acts as a base in your blood, helping neutralize excess acid.

Why does pH matter so much? Enzymes are highly sensitive to pH. Each enzyme has an optimal pH range, and even small deviations can change the enzyme's shape and shut down its activity. If blood pH drifts outside its normal range of 7.35–7.45, cellular processes start to fail, which is why your body works hard to maintain acid-base homeostasis.

Buffer Systems for pH Homeostasis

A buffer system resists changes in pH when small amounts of acid or base are added. Buffers consist of a weak acid paired with its conjugate base (or a weak base with its conjugate acid). Your body relies on three main buffer systems:

1. Bicarbonate buffer system — the primary buffer in extracellular fluid. It pairs carbonic acid ($H_2CO_3$) with bicarbonate ion ($HCO_3^-$). Your respiratory system regulates this by adjusting how much $CO_2$ you exhale, and your kidneys fine-tune it by excreting or reabsorbing $HCO_3^-$.

2. Phosphate buffer system — the major intracellular buffer. It pairs dihydrogen phosphate ($H_2PO_4^-$) with hydrogen phosphate ($HPO_4^{2-}$). This system is especially important inside cells and in urine.

3. Protein buffer system — amino acids can act as either weak acids or weak bases depending on the pH of their environment. Hemoglobin is a particularly important protein buffer in the blood, helping to stabilize pH as it picks up and releases $CO_2$ and $H^+$.

Water as a Solvent in Cellular Processes

Water's solvent properties are what make cellular chemistry possible. Nutrients, gases, and waste products all need to be dissolved in water to be transported and used by cells.

• Hydrophilic substances (ions, sugars, many proteins) dissolve readily in water, which is how they move through blood plasma and into cells.
• Hydrophobic substances (fats, oils) don't dissolve in water. This property is actually useful: it's what allows phospholipids to form the cell membrane, creating a barrier between the watery interior and exterior of the cell.

Water also participates directly in chemical reactions. In hydrolysis reactions, water is used to break bonds in large molecules. In dehydration synthesis, water is removed to build larger molecules from smaller subunits. These reactions are central to metabolism, from digesting food to building proteins.